Very large scale integrated circuit (VLSIC) is reputed as the industrial fodder for the importance in modern world. The lithographic tool is the kernel piece of the equipment for the VLSIC manufacture, and its progress directly enables the decrease in size of the VLSIC and increase in integration scale; whether the feature size of the integrated circuit (IC) continues to decrease follow the “Moore's Law” is dependent on the development of lithography technology. The history of the continual decrease in the feature size of the IC is also a history of the continual evolution of lithographic tools. Lithographic exposure wavelength continues to decrease from the g line (436 nm) and I line (365 nm) of a mercury lamp to the 248 nm and 193 nm of an excimer laser. The exposure mode of a lithographic tool has experienced three stages: contact lithography, proximity lithography, and optical projection lithography, with projection lithography being the exposure mode for almost all high-end lithographic tools nowadays. The projection lens system is the kernel sub-system of a projection lithographic tool and the key to lithographic resolution. While the aberration is a major property parameter for a projection lens, the smaller the aberration, the better the imaging quality. A lithographic projection lens has an optical component for adjusting aberration, and the basis for adjustment is dependent on measurement result of aberration of the projection lens. Hence, the level of control of aberration of a projection lens relates directly to the measurement precision of aberration.
A 193 nm projection lens has combined with multi patterning techniques in realizing scale production of 14 nm node integrated circuits, and is very likely to extend to 10 nm node and 7 nm node. To increase lithographic production rate, the power of the light source continues to increase, and under the multi pole or free form illumination; thermal effect of the projection lens ever increases, and the issue of thermal aberration is getting evermore prominent. To control the imaging quality more precisely, the measurement of the aberration of the projection lens shifts from lot-by-lot measurement to wafer by wafer measurement. In order not to decrease the production rate, the measuring speed of aberration has to increase. Aberration of a projection lens is field point related, that is, different field points have different aberration, and thus measurement of wave aberration data for multi field points is required. To increase measurement speed, multi field point parallel detection can be adopted.
Shearing interferometer is structurally simple, requires no reference wave, is easy to integration, and thus employed as an in situ detection technique for aberration. DE 10008181 discloses a shear interferometer for lithographic aberration detection, where a one-dimensional grating is disposed on an object plane of a projection lens, and a two-dimensional grating is disposed on an image plane thereof; the object plane one-dimensional grating modulates spatial coherence of the optical field, while the image plane two-dimensional grating is employed for shearing the wavefront under test, with a detector arranged in a defocusing position for obtaining an interferogram of the under test wavefront and the sheared wavefront. Such a method generates the phase shift by means of mechanically moving the object plane grating or the image plane grating; in the case of generating phase shift by means of moving the object plane grating, the position of the measured field point moves as the grating moves; in the case of generating phase shift by means of moving the image plane grating, the measured pupil position deviates; regardless whether it is the position of the measured field point or the pupil position which deviates, a measurement error results. In addition, such a method needs to replace the grating for a different grating line direction when shear direction is changed.
U.S. Patent Application Publication 2006/0001890 A1 discloses an improved shear interferometer for aberration detection of a lithographic projection lens by adopting a spatial light modulator on the object plane as an object plane grating generator. Phase shift is generated by means of digitally moving the gratings of the spatial light modulator, thus avoiding the issue of the moving of the detected field points; in addition, digitally changing the grating line direction of the gratings of the object plane spatial light modulator is more convenient than replacing gratings. For a specific detected wavefront and a projection lens, the shear ratio is dependent on the image plane gratings, thus DE 10008181 and U.S. Patent Application Publication 2006/0001890 A1 both require to replace the image plane gratings in order to change shear ratio. Aberration detection is sensitive to different shear ratio, and therefore there is a need in aberration detection to change shear ratio.
WO 2014/060149 A1 discloses a multi channel parallel detection shear interferometer for multi field point aberration parallel detection for a lithographic projection lens. A grating board comprising n groups of gratings is disposed on the object plane of the projection lens, where n is the number of parallel detection field points, with each group of gratings comprising two one-dimensional gratings, the grating line direction thereof being perpendicular to one another, for two shear measurements on two mutually orthogonal directions; a grating board comprising n two-dimensional chessboard gratings is disposed on the image plane, the diagonal directions of the chessboard grating being parallel to the grating line directions of the one-dimensional gratings on the object plane. In actual detection, adjust the object plane gratings and the image plane gratings so that their positions satisfying an object-image relationship, with n detectors being placed on a defocusing position under the image plane gratings for reception of the wavefront shear interferogram of each field point. The method requires mechanically moving the object plane gratings or the image plane gratings to generate phase shift, as well requires mechanically moving the grating boards to replace gratings in the other direction for changing shear directions Because shear ratio is restricted by the period of the image plane gratings, so the object plane gratings and the image plane gratings need to be changed at the mean time to change shear ration.